Some of the big - issues with shoulder holster carry include the slow re-holstering(this is even worse in high stress or critical incidents), concealment & firearm retention.
A shoulder rig may work best for a low threat area or for use while driving or seated for long periods.
A few leather models like the Miami Classic II & the Kirkpatrick design have canted mag holders. These work well, IMO, because they free up space on the belt. This allows space for a BUG or 2nd gun, like a smaller pistol or J frame revolver.
Galco marketed a slick off side magazine holder that could hold 4 or 5 fully loaded magazines but Im not sure if it's still for sale.
I think it was meant for sworn LE agents or plainclothes PSD/PMCs(military contractors) in SW Asia(Iraq-OEF).